The present invention relates to a method of forming a thick film circuit pattern. In particular, the present invention relates to a method of forming a thick film circuit pattern with a sufficiently wide and uniformly thick film strip applied at a high speed with precision.
Conventional thick-film forming methods are broadly classified into screen printing method and nozzle drawing method. The screen printing method involves preparation of a new screen each time a new circuit pattern is developed. While screen preparation takes a substantial amount of time, the screen method allows highly efficient printing and for this reason this method has been extensively used in mass production. In the trail stage of development where repeated attempts are made to obtain a desired circuit pattern, the drawing method is employed to take its advantage in that it allows full utilization of the capability of a computer to draw a circuit pattern according to a program that can be easily changed.
As illustrated in FIG. 1, conventional drawing methods involve the use of a drawing nozzle 3 having a circular opening through which thick-film paste 1 is ejected onto a substrate 2 and the nozzle 3 is moved relative to the substrate to form a desired circuit pattern. Nozzles of this type are particularly useful for producing a pattern having a small width in the range from 25 micrometers to 150 micrometers. As shown in FIG. 2 where a typical example of the nozzle 3 is illustrated, the nozzle opening usually has a diameter in the range from 50 micrometers to 200 micrometers. However, a measurement made by a surface roughness meter after baking revealed that the cross-section of the deposited film pattern took the shape of a semicircle having a large thickness (see FIG. 3) at the center. This thickness increases as the diameter of the drawing nozzle increases, making it difficult to deposit a film strip with a wide and uniform thickness. In the case of a resistive paste used for depositing resistors, large thickness makes it difficult to trim the resistance value. Use of a high power laser would produce microscopic cracks in the crystalline structure of the film which might result in a gradual change in resistance value and could lead to the loss of device reliability. For most applications the required cross-section of resistive film strips is 12 micrometers or less in thickness and about 1.0 millimeter in width. However, use of a circular nozzle that meets the thickness requirement of less than 12 micrometers would only result in a strip with a width of only 150 micrometers. The current practice thus involves forming closely spaced parallel strips until a desired width is attained. However, this is time consuming and often results in nonuniform thickness causing film-to-film variations.
Another prior art method employing the drawing nozzle 3 is shown in FIGS. 4 and 5 in which a stylus 5 is attached to the tip of the nozzle. The nozzle is moved with the stylus 5 in contact with the surface of the substrate 2 so that the nozzle opening follows the contour line of the surface irregularities of the substrate. Alternatively, the nozzle 3 is tilted in a manner shown in FIG. 6 and moved to the right in order to follow the surface contour of the substrate. Although these methods have proved successful to achieve uniformity in thickness, the surface of the substrate is impaired as shown at 6 in FIGS. 6 and 7 along the path of the moving stylus and nozzle end and such impairment affects the electric characteristics of the resistance measured after the pattern is baked. Another disadvantage of these methods resides in the fact that since the substrate is formed of a hard material such as ceramic, the stylus and the nozzle end are worn out by contact therewith requiring frequent replacement and that the nozzle speed is limited by their relatively poor rigidity. Because of the various disadvantages noted above, the prior art drawing nozzle is not applicable to mass production.